Effects of Medial Septal Lesions
The modern reformulation (O’Keefe & Nadel, 1978) of Tolman’s (1948) cognitive map theory has largely been the result of two major findings which have provided the impetus for the direction of research over the last 2.5 decades; viz., rats with hippocampal lesions are deficient in negotiating such spatial memory tasks as the radial arm maze (Olton, and Samuelson, 1976) and the water maze (Morris, Garrud, Rawlins & O’Keefe, 1982); and the discovery of place cells in the hippocampus which fire when the rat is in a specific location or place field (O’Keefe, 1976). The cognitive map theory of hippocampal functioning suggests that this structure processes spatial information detailing the rat’s current position in the environment and provides the necessary computational skills allowing movement to a target goal (O’Keefe, 1989). The major neurotransmitter afferents to the hippocampus from the medial septal area and the associated vertical limb of the diagonal band of Broca (MSDB) include acetylcholine (Amaral & Kurz, 1985) projecting to widely distributed areas, and GABA (Freund & Antal, 1988) which innervate most of the GABAcontaining interneurons in the hippocampus. MSDB GABA also projects inhibition to the lateral septal area which in turn receives the output of the hippocampal glutamatergic pyramidal cells. Some of these cells additionally terminate in MSDB. Lateral septal neurons do not, as previously suggested project to MSDB (Leranth, Deller & Buzsaki, 1992) but do project reciprocally to the hypothalamus which returns afferents to the hippocampus and MSDB.
KeywordsWater Maze Entorhinal Cortex Reversal Learning Place Cell Theta Rhythm
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